The most underrated act of kindness is simply letting people be. Let them mispronounce a word, talk too much about a show they love, or get excited about something you don't quite understand. Everyone has something that lights them up, let them shine, even if it's not your thing.
One thing l've learnt is that people know what they are doing. They know how it affects you, they know how it would feel if it's done to them, they know.